1. Field of the Invention
The present invention relates to an optical fiber amplifier, and more particularly to a wideband erbium-doped optical fiber amplifier for amplifying C-band and L-band optical signals.
2. Description of the Related Art
An optical fiber amplifier is an apparatus used in an optical transmission system to amplify optical signals without optoelectric conversion. Accordingly, the optical fiber amplifier has a simple and economic construction. Such an optical fiber amplifier includes (1) a gain medium optical fiber, (2) a pumping light source necessary in optical pumping, (3) a wavelength division multiplexing (WDM) optical coupler for coupling an optical signal and pumping light to the gain medium optical fiber, and (4) an optical isolator for passing forward light and intercepting backward light.
The optical signal is amplified through an induced discharge of rare-earth elements, such as erbium, added to the gain medium optical fiber. Specifically, the pumping light excites the rare-earth element ions added to the gain medium optical fiber. Thereafter, the optical signal incident to the gain medium optical fiber is amplified through the induced discharge of the excited ions. In current ultrahigh speed WDM optical transmission systems, a wavelength band of 1.55 μm is widely used along with erbium-doped optical fiber amplifiers suitable for amplifying such a wavelength band. WDM optical technology is capable of simultaneously transmitting a plurality of channels with different wavelengths using a single-core optical fiber. WDM optical technology, researches are actively seeking wider transmission bands, for example by using optical signals not only the C-band, having a wavelength band of 1525 to 1565 nm, but also the L-band, having a wavelength band of 1570 to 1610 nm. In particular, researchers are seeking a wideband erbium-doped optical fiber amplifier (which is one of core elements of a WDM optical communication system) that can amplify not only C-band optical signals but also L-band optical signals.
A typical C-band erbium-doped optical fiber amplifier utilizes a population inversion of 70 to 100%. This produces non-uniform gain characteristics (according to wavelengths) for the C-band erbium-doped optical fiber amplifier. Usually, the C-band erbium-doped optical fiber amplifier has the highest gain at a wavelength of 1530 nm and has the lowest gain at a wavelength of 1560 nm. Various gain-flattening methods are used, since the C-band erbium-doped optical fiber amplifier has non-uniform gain characteristics. Conventional gain-flattening methods include a method employing an optical filter, a method employing a Fabry-Perot filter, a method employing a Mach-Zender interferometer, a method employing a dielectric thin film, and a method employing an optical Fiber Bragg Grating (FBG), etc. In such gain flattening methods, a filter designed to have a loss spectrum that is opposite to the gain spectrum of the C-band erbium-doped optical fiber amplifier is used, thereby obtaining a uniform gain regardless of wavelengths. Among the various gain flattening methods described above, the method employing an optical Fiber Bragg Grating is generally utilized.
An optical fiber grating is an optical fiber element having optical fiber cores each of which has a periodically changing refractivity. They either reflect or eliminate optical signals (channels) of specific wavelengths from multi-wavelength optical signals incidented to the optical fiber grating. Optical fiber gratings may be classified into long period (reflection type) and short period (elimination type) optical fiber gratings. In the short period optical fiber grating, optical fiber cores have a refractivity changing in a period of several hundreds nanometers (which is generally called “grating period”). Optical fiber mode coupling occurs between a forward mode and a backward mode, thereby reflecting only a channel of a specific wavelength from an incidented multi-wavelength optical signal. In contrast, in the long period optical fiber grating, a grating period is several hundreds micrometers. Optical fiber mode coupling occurs between two forward modes, thereby eliminating only a channel of a specific wavelength from an incidented multi-wavelength optical signal. A transmission (reflection) spectrum of an optical fiber grating can be properly adjusted according to the grating period, grating intensity, grating length, and refractivity distribution.
In one method of employing a long period optical fiber grating for flattening the gain of the C-band erbium-doped optical fiber amplifier, the long period optical fiber grating is first designed to have a transmission spectrum opposite to the gain spectrum of the C-band erbium-doped optical fiber amplifier. Then it is inserted into the C-band erbium-doped optical fiber amplifier, thereby enabling the gain to be uniform regardless of the wavelengths. This method does not require a separate additional optical element since there is no reflected optical signal. However, this method has a number of shortcomings including having a spectrum characteristic that is very sensitive to temperature. In order to overcome such temperature sensitivity, another method employing a chirped optical fiber grating (or Chirped Fiber Bragg Grating; CFBG) has been proposed. This method has a short period optical fiber gratings. The CFBG has a grating with a grating period that changes linearly or non-linearly in a longitudinal direction of the grating. In this method, the CFBG is designed with a reflection spectrum opposite to the gain spectrum of the C-band erbium-doped optical fiber amplifier. Then, it is inserted into the erbium-doped optical fiber amplifier, thereby enabling the gain to be uniform. However, this method requires an additional optical element such as an optical isolator in order to prevent an optical signal reflected by the CFBG from coupling and interfering with a forward optical.
When compared to a C-band erbium-doped optical fiber amplifier, an L-band erbium-doped optical fiber amplifier shows no difference in the pumping light source.
However, it is about five to ten times longer, since the L-band erbium-doped optical fiber amplifier utilizes population inversion of about 40%. Further, an article entitled “Flat gain erbium-doped fiber amplifier in 1570 nm–1600 nm region for dense WDM transmission systems”, OFC '97, vol. PD3, 1997, by M. Fukushima, Y Tashiro, and H. Ogoshi, has shown that the gain flattening characteristic of an L-band erbium-doped optical fiber amplifier is improved through co-pumping by auxiliary pumping light source of the C-band (1530, 1550, or 1570 nm) wavelength together with an existing high power LD light source of 980 or 1480 nm However, such a method requires a separate exterior light source as an auxiliary pumping light source.
FIG. 1 illustrates a conventional wide band erbium-doped optical fiber amplifier. The conventional erbium-doped optical fiber amplifier 100 is disposed on an external optical fiber 110 and includes a first and a second amplifying section 170 and 180 and a first and a fifth WDM coupler 121 and 125 for connecting the first and second amplifying section 170 and 180 in parallel to each other.
The first WDM coupler 121 divides an optical signal of 1550 and 1580 nm wavelength bands received through the external optical fiber 110 into optical signals of a 1550 nm wavelength band (C-band) and a 1580 nm wavelength band (L-band). Then it outputs the C-band optical signal to a first optical path and the L-band optical signal to a second optical path.
The first amplifying section 170 includes a first and a second isolator 131 and 132, a first pump LD 141, a second WDM coupler 122, a first erbium-doped optical fiber 151, and a chirped optical fiber grating 160. Each of the first isolator 131 and the second isolator 132 intercepts backward light such as Amplified Spontaneous Emission (ASE) noise outputted from the first erbium-doped optical fiber 151. The first pump LD 141 outputs a first pumping light having a wavelength of 980 nm or 1480 nm. The second WDM coupler 122 is interposed between the first isolator 131 and the second isolator 132. It couples the C-band optical signal having passed the first isolator 131 with the first pumping light inputted from the first pump LD 141. Then, it outputs the coupled light. The first erbium-doped optical fiber 151 experiences a population inversion (is pumped) by the first pumping light that has passed the second isolator 132. It also amplifies the C-band optical signal that has passed the second isolator 132. The chirped optical fiber grating 160 gain-flattens the C-band optical signal received from the first erbium-doped optical fiber 151.
The second amplifying section 180 includes a third isolator 133, a second and a third pump LD 142 and 143, a third and a fourth WDM coupler 123 and 124, and a second erbium-doped optical fiber 152. The second pump LD 142 intercepts backward light such as ASE noise outputted from the second erbium-doped optical fiber 152. The second pump LD 142 outputs a second pumping light having a wavelength of 980 nm or 1480 nm. The third WDM coupler 123 couples the L-band optical signal that has passed the third isolator 133 with the second pumping light received from the second pump LD 142. Then it outputs the coupled light. The third pump LD 143 outputs a third pumping light having a wavelength of 1550, 1530 or 1570 nm. The fourth WDM coupler 124 couples the L-band optical signal inputted from the third WDM coupler 123 with the second and third pumping lights. Then it outputs the coupled light. The second erbium-doped optical fiber 152 experiences a population inversion by the second and third pumping lights received from the fourth WDM coupler 124. It also amplifies the L-band optical signal received from the fourth WDM coupler 124.
The fifth WDM coupler 125 couples the C-band and L-band optical signals received through the first and second optical paths. Then it outputs them through the external optical fiber 110.
The first erbium-doped optical fiber 151 and the second erbium-doped optical fiber 152 have similar construction. The second erbium-doped optical fiber 152 has a length larger than that of the first erbium-doped optical fiber 151. Further, each of the first and second erbium-doped optical fibers 151 and 152 has a forward pumping construction in which the received optical signal and the pumping light progress in the same direction. However, each of them may have a backward pumping construction in which the inputted optical signal and the pumping light progress in opposite directions, if necessary.
As described above, the conventional wideband erbium-doped optical fiber amplifier 100 has gain flattening characteristics of not only the C-band but also the L-band optical signal. However, the conventional wideband erbium-doped optical fiber amplifier 100 has a number of limitations, including (1) that the first amplifying section 170 must include the second isolator 132 which is an additional element for preventing generation of backward ASE noise and (2) the second amplifying section 180 requires the second pump LD 142 as a separate and auxiliary pumping light source.